1. Field of the Invention
The present invention relates to an optical recording/reproducing apparatus for recording or reproducing information on/from a recording medium.
2. Description of Related Art
Conventionally, a number of optical recording/reproducing apparatuses using a semiconductor laser have been developed. Among them, an optical recording/reproducing apparatus using an optical disk as a recording medium is receiving attention because a further increase in capacity and speed can be expected. Such an optical recording/reproducing apparatus emits a laser beam onto a surface of an optical disk so as to form a smaller spot, whereby the recording density of the optical disk is increased and a higher capacity can be achieved. Examples of an optical recording/reproducing apparatus using such a technique include an optical recording/reproducing apparatus that uses a Blu-Ray Disc as an optical disk and uses a semiconductor laser as a light source that emits a blue-violet laser beam having a wavelength in the vicinity of 405 nm to the optical disk, and such an optical recording/reproducing apparatus is being developed actively.
However, when information on an optical disk is reproduced by using a laser beam having a short wavelength in the vicinity of 405 nm as above, a smaller spot formed on a surface of the optical disk receives a higher irradiation energy. Therefore, problems may arise that the optical disk is deteriorated due to a rise in temperature, that recorded information is erased, and the like. To solve these problems, when the output power of a semiconductor laser is reduced so as to reduce the irradiation energy on a spot on the surface of the optical disk, then another problem arises that the signal-to-noise ratio (S/N) is degraded due to the quantum noise of the semiconductor laser itself.
As a solution to the above-mentioned problems, JP 2000-195086 A, for example, describes an optical recording/reproducing apparatus as a first conventional example in which an intensity filter is inserted between a semiconductor laser and an optical disk, so that the intensity (irradiation energy) of a laser beam on a surface of the optical disk is attenuated without a reduction in the output power of the semiconductor laser.
However, the optical recording/reproducing apparatus according to the first conventional example cannot record/reproduce information on/from both a recording medium having a single recording layer and a recording medium having a plurality of recording layers. In order to solve this problem, an optical recording/reproducing apparatus is proposed that changes the transmittance of an intensity filter so as to record/reproduce information also on/from a recording medium having a plurality of recording layers (see, for example, JP 2003-115109 A).
The optical recording/reproducing apparatus described in JP 2003-115109 A will be described as a second conventional example. FIG. 18 is a block diagram showing a configuration of the optical recording/reproducing apparatus according to the second conventional example.
As shown in FIG. 18, the optical recording/reproducing apparatus according to the second conventional example includes an optical head 120, a filter driving unit 130, a laser power control unit 131, an output power detection unit 132, a focus control unit 133, a tracking control unit 134, a reproduction signal processing unit 135, a medium judgment unit 136, and a control unit 137. The optical head 120 includes a semiconductor laser 101, a collimator lens 102, an intensity filter 103, a beam splitter 104, a quarter-wave plate 105, an objective lens 106, detection lenses 108 and 110, and light-receiving elements 109 and 111.
In the optical recording/reproducing apparatus according to the second conventional example, the filter driving unit 130 inserts or removes the intensity filter 103 in/from a path (hereinafter, referred to as an optical path) of a laser beam emitted from the semiconductor laser 101 in accordance with the number of recording layers of the optical disk 107, which is judged by the medium judgment unit 136. In other words, the arrangement of the intensity filter 103 in the optical path is changed depending upon the number of recording layers of the optical disk 107.
Hereinafter, an operation of the optical recording/reproducing apparatus according to the second conventional example will be described.
First, a recording layer will be described with reference to FIG. 20.
FIG. 20A is a cross-sectional view showing an example of the optical disk 107. FIG. 20B is a cross-sectional view showing another example of the optical disk 107.
FIG. 20A shows a single-layer disk having a single recording layer 107a. FIG. 20B shows a dual-layer disk having two recording layers 107b and 107c. Herein, it is assumed, for example, that the reproduction power of a laser beam required to reproduce information from the single-layer disk is 0.4 mW, that the optical transmittance (transmittance of the entire optical system of the optical head excluding the intensity filter 103) of the optical head 120 is 25%, and that the optical transmittance of the intensity filter 103 that is inserted in or removed from the optical path is 50%.
When the optical disk 107 has a single-layer structure as shown in FIG. 20A, it is appropriate that the intensity filter 103 is inserted in the optical path. When the intensity filter 103 is inserted in the optical path, the reproduction power of the laser beam is attenuated to 50%. Accordingly, the output power of the semiconductor laser 101 can be set to 3.2 mW (0.4 mW/25%/50%), and the quantum noise of the semiconductor laser 101 can be suppressed within an allowable range.
On the other hand, when the optical disk 107 has a dual-layer structure as shown in FIG. 20B, it is appropriate that the intensity filter 103 is removed from the optical path. When the intensity filter 103 is removed from the optical path, the transmittance with respect to the laser beam is 100%. The reproduction power required to reproduce information from the dual-layer disk is about twice as high as that for the single-layer disk. This is because the transmittance of the recording layer (recording layer 107c in FIG. 20B) located closer to the objective lens 106 out of the two recording layers of the optical disk 107 is set at about 50%. That is to say, the reproduction power of the semiconductor laser 101 required to reproduce information from the dual-layer disk is 0.8 mW (0.4 mW×2). Accordingly, the output power of the semiconductor laser 101 is 3.2 mW (0.8 mW/25%), and the quantum noise of the semiconductor laser 101 can be suppressed within an allowable range.
In the above example, the output powers of the semiconductor laser 101 required to reproduce information from the single-layer disk and from the dual-layer disk are both 3.2 mW. Further, assuming that the recording power of the laser beam required to record information on the single-layer disk is 6 mW, the output powers of the semiconductor laser 101 required to record information on the single-layer disk and on the dual-layer disk are both 48 mW (6 mW/25%/50%=12 mW/25%). As described above, the optical recording/reproducing apparatus according to the second conventional example is capable of reproducing or recording information from/on the optical disks 107 having a different number of recording layers without changing the output power of the semiconductor laser 101.
Next, an operation of the optical recording/reproducing apparatus according to the second conventional example will be described with reference to FIG. 19.
FIG. 19 is a flow chart showing an exemplary starting procedure for the optical recording/reproducing apparatus according to the second conventional example.
Initially, the optical disk 107 is inserted (Step S501). Then, the medium judgment unit 136 judges the number of recording layers of the optical disk 107 (Step S502). When the optical disk 107 has a single recording layer, the filter driving unit 130 inserts the intensity filter 103 in the optical path (Step S503). When the optical disk 107 has two recording layers, the filter driving unit 130 removes the intensity filter 103 from the optical path (Step S504). Thereafter, the reproduction signal processing unit 135 makes various adjustments in accordance with the type of the disk, and starts recording or reproducing information on/from the optical disk 107 (Step S505).
However, in the optical recording/reproducing apparatus according to the second conventional example, the state of the intensity filter 103 (transmission amount variation unit) is not detected. Thus, when the intensity filter 103 is arranged in a state different from the instruction from the control unit 137 due to a malfunction of the intensity filter 103 or the filter driving unit 130 (transmission amount variation unit), such a misarrangement cannot be detected.
Problems that arise when the intensity filter 103 is inserted in or removed from the optical path mistakenly will be described below. Herein, it is assumed that the optical transmittance of the optical head 120 (excluding the intensity filter 103) is 25%, for example.
For example, in the case where the optical disk 107 has a single-layer structure and the intensity filter 103 is not inserted in the optical path, in order to obtain a reproduction power of 0.4 mW, the output power of the semiconductor laser 101 is required to be set to 1.6 mW (0.4 mW/25%). In such a case, the semiconductor laser 101 suffers high quantum noise beyond its acceptable level. Therefore, a sufficient signal-to-noise ratio (S/N) may not be obtained.
On the other hand, for example, in the case where the optical disk 107 has a dual-layer structure and the intensity filter 103 is not removed from the optical path, assuming that the recording power required to record information on the single-layer disk is 6 mW, the recording power required to record information on the dual-layer disk is 12 mW (6 mW×2), which is about twice as high as that for the single-layer disk. Assuming that the optical transmittance of the intensity filter 103 left in the optical path is 50%, the semiconductor laser 101 is required to emit light with an output power of 96 mW (12 mW/25%/50%) in consideration of the transmittance, which is twice as high as that in the case where the intensity filter 103 is removed. Accordingly, a current larger than allowed is supplied to the semiconductor laser 101, and therefore the semiconductor laser 101 may be deteriorated or destroyed.